Rheostat



DBC. 2.0, 1932. I Q, W, GREENE 1,891,410

RHEOSTAT Filed Sept. ".50, 1927 HIS TORNEYS Patented Dee 2o, 1932 UNITED STATES OTTO W. GREENE, F ELYRIA, OHIO RHEOSTAT Application led September 30, 1927. Serial No. 223,002.

This invention relates to a rheostat and is particularly suitable for use where highly varying resistances are desired and' Where lminute and gradual changes in resistance are advantageous.

Heretofore rheostats have been made in which a carbon pile is made use of, that is, a series of plates of carbon or like high resistance material were used as the rheostat 10 and varyin pressure applied to the ends of the .stack o? plates to cause the resistance to vary, the resistance being decreased upon increased application of pressure to the pile, which increased pressure forced the minute irregularities upon the contacting surfaces of the plates into a more intimate Contact and so produced a more eiiicient electrical con nection between the adjacent plates. Rheostats of this type have certain disadvantages in that large and bulky apparatus is necessary in handling ordinary voltages on account of the low resistance variation per pair of contacting surfaces. Moreover on releasing the pressure upon the rheostat to increase its resistance, certain ofthe plates would tend to stick together so that the resistance lbetween these did not increase as desired but the resistance betweenthose plates which did separate readily had to be increased far be- '30 yond that anticipated, and this, at the relatively high voltages used, caused arcing at these points, which pitted the surfaces of the plates and reduced their eiiiciency as conductors. Moreover this tendency to arc necessitated enclosing the carbon piles within insulating housings and so interfered with the dissipation of heat therefrom, rendering them unsuitable for long operation.

It is an object of the present invention to provide, a rheostat in which the high resistance properties of carbon or thelike may be utilized without acquiring the disadvantages attendant to prior rheostats using such materials.

The invention will be better understood from the description of one practical embodiment-thereof illustrated in the accompanying drawing, in which Figure 1 is a front elevation of a rheostat 5 embodying the invention.

Figure 2 is a bottom plan view thereof,

Figure 3 is a side elevation.

Figures 4, 5 and 6 are front elevations of one pair of adjacent plates, subjected to different degrees of pressure, and

Figures 7, 8 and 9 are perspective views of pairs of modilied forms of resistance plates.

Referring irst to Figures 1 to 3, the apparatus comprises a frame having a back plate l and an outwardly extending flange or abutment 2 formed integrally therewith at the top. The frame is secured to a support such as a wall by screws or bolts 3 and carries, upon a pivot 4, a pivoted bell crank or lever having a. handle 5v and actuating arm 6. A lug 7 formed integrally with'the back plate is provided with a threaded aperture in which a set screw 8 is positioned to limit the movement of the handle so that the actuating arm will not move below a desired position. Interposed between the arm 6 and the iange 2 are a series of plates of carbon or similar resistance material, each of which may be conveniently cut from a rectangular s trip by sawing long V-shaped slots 9 into the ends thereof in a transverse or horizontal direction, and other V-shaped slots 10 in a verti cal direction. The adjoining faces of these resistance blocks are slightly concave, the conu cavity being shown exaggerated for purposes of illustration only, so that the adjoining carbon blocks contact each other normally merely. at their four outer corners 11. A contact or pressure applying block 12 is interposed between arm 6 and the lowermost resistance block and the uppermost and lowermost resistance blocks are provided with conduits 13 and 14, which may be molded into the same. When the handle 4 is moved to the right as shown in Figure 1, arm 6 presses the contact block 12 upwardly, pressing the resistance blocks 8 together, the manner in which this takes place being most clearly shown in Figures 4 to 6, in which Figure 4 shows the adjacent surfaces of two blocks in the position which they occupy when substantially no pressure is applied, pressing only at their outer ends 11 and separated by a lenticular shaped space 15,. In this position, it will be noted that the area of contact between the plates is very small, that the electricity will follow the path indicated by the arrows out to the very end of one p ate, then across to the adjacent Plate and back to substantially center thereof, and it will also be noted that the contacts at the end, being made under very li ht pressure, are very ineflicient and that t e minimum cross section of the carbon through which the current must pass is that just adjacent the tips of the blocks, at about 16. In Figure 5 the blocks have been partially pressed together and it will be noted that the area of contact at the ends now extends from the extreme tip to the point marked 17, the path of the current being again as indicated by the arrows, which will be seen to be not much more than half. of what it is when the block is in the position shown in Figure 4 and the smallest cross section of carbon througxh which it must pass is that just adjacent to t e point 17, the carbon at this point being appreciably thicker and wider than at the oint 16 above referred to. In Figure 6 the b ocks have been pressed entirely together and the current may now flow directly therethrough, traversing a path merely equal to the thlckness of the blocks. The area of contact between the surfaces of adjacent blocks is much reater than the smallest cross section of the lock itself, and of course a much more efiicient contact per unit area has been established between the .surfaces by the increased pressure.

When pressure upon the blocks is released, each block acts as a small semi-elliptic spring to force itself into the position shown in Fi ure 4, preventing any of the plates from stic ing together substantially more than others, and so not allowing undue increase of voltage drop across any two adjacent surfaces. The concave surfaces may be conveniently formed by first forming the notches and then grinding the contacting surfaces asV though they were to be flat, the ends being allowed to flex away from the grinding wheel,

From the above it will be seen that the resistance of the rheostat is increased in three wa s when the handle is moved over:

12 The length of the path through the car n which the current must traverse is decreased as the surfaces flatten upon each other.

(2) The minimum cross section area of the carbon path of the current is increased as the blocks are pressed together, and

(3) Eiliciency of the contact between the adjacent blocks increased with increased pressure.

By these means a very high variation in resistance may be obtained with very few resistance blocks, while at the same time the resistance is smoothly and uniformly varied as the blocks are pressed together or allowed to spring apart. Arcing does not take place and the resistance elements need not, therefore, be enclosed, but are preferably exposed to the air so that heat generated within them will readily be dissipated.

In Figure 7, a pair of blocks are shown which are very similar to those above described except that they are uivalent to only a half of each of the above locks, such as would be formed if the blocks were cut on horizontal planes throu h their centers. The operation of these bloc s is identical to that ust described except that twice as many locks must be used, the outer smaller surfaces of one pair Contactin with those of another and possessing merey the properties 3 of the older carbon piles, the resistance of which increases only as the elliciency of the contact is increased b ressure.

In Figure 8 a (pair o blocks is shown which are circular or isc shaped, the adjacent sur- 135 faces of the blocks being slightly conical or spherical in curvature so that they touch at t e edges only until pressure is ap lied to bend them in the manner described a ve.

In Figure 9, circular blocks such as shown in Figure 8 have been provided with radial slits to render the edges thereof more flexible, the operation otherwise being exactly the same.

From the above it will be seen that many variations and modifications will readily occur to those in the art to which this invention appertains, without departing from the spirit of the invention, and I do not, therefore, llmit myself to the recise details shown and described, but claim as my invention all the ernbodiments falling within the scope of the subjoined claims.

I claim Y 1. In a rheostat to control the flow of current according to pressure, carbon blocks each having a portion with a conductor area for carrying the full current flow, flexible ortions integral therewith, the flexible portions of adjoining blocks having non-mating opposed surfaces, the flexible portions of each lock being separated by a rtures extending therebetween, the flexiblleportions being provided with a ertures normal to the contact surfaces an opening extcriorly thereof.

2. In a rheostat for continuously control-l ling a wide range of current capacities, carbon blocks having normally non-mating surfaces localized portions of which contact under light pressure whereby a circuitous current mth through the blocks is obtained, means or pressing the blocks to ether to vary the surfaces in contact an thereby progressively reduce the len h of the current path, the carbon blocks ing provided with relieved ortions extending inwardly to the portion of) the blocks throu h which the current flows in its shortest pat to form an aperture for entrance of air.

3. In a rheostat for continuously controlling a wide range of current capacities, carbon blocks having normally non-mating surfaces localized portions of which contact under light pressure whereby a circuitous current path through the blocks is obtained, means for pressing the blocks together to vary the surfaces in contact and thereby pro gressively reduce the length of the current path, the carbon blocks being provided with relieved portions extending inwardly to the portion of the blocks through which the current flows in its shortest path to form an aperture for entrance of air, the blocks being provided with slots normal to the contact faces extending through the blocks to combine with the relieved space for entrance of air and to reduce the area in contact at light pressures.

4. In a rheostat for continuously controlling a wide range of current capacities, carbon blocks having portions with mating surfaces for the conductance of full current flow, and portions having normally non-mating surfaces making contact under light pressure at localized portions thereof whereby a circuitous current path through the blocks is obtained, means for pressing the blocks together to vary the 'surfaces in contact Whereby the current path is progressively reduced, the carbon blocks being provided with relieved portions extending substantially parallel with the faces and reaching inwardly to the portion thereof through which the maximum current flows, the blocks being provided With slots normal to the Contact faces whereby, the contact area is reduced and en trance .of air permitted.

5. A rheostat to control the flow of current according to pressure comprising blocks of resistance material, each havinga body portion with a conductor area for carrying full current flow, portions integral therewith having non-mating opposed surfaces in adjoining blocks, the last mentioned portions in each block being separated by slots whereby the non-mating surfaces can be brought into v contact progressively upon application of throughout their length, the blocks having pressure, said portions being provided with apertures normal to vthe contact surfaces lwhereby the surfaces in contact under light pressure are small, the apertures opening to the atmosphere.

6. A rheostat comprising blocks of resistance material having contact surfaces normally non-mating and contacting only at their ends but adapted to be pressed into full contact by pressure, said contact surfaces having a varying radius of curvature whereby an even and progressive increase in area. of contact is obtained upon application of pressure.

7. A rheostat comprising blocks of resistance material having varying sectional area opposed concave contact faces normally nonmating but adapted to be pressed into full contact by pressure, said contact faces having a varying radius of curvature along the block whereby an even and progressive in crease of area in contact can be obtained upon application of pressure.

8. A rheostat comprising carbon blocks, the adjoining surfaces of which are concave whereby they make contact normally only at their ends, the blocks decreasing in iiexlbility from their ends inwardly, and means for selectively pressing the blocks together.

-9. A rheostat comprising ,carbon blocks disposed in abutting relation and provided with concave adj oinmg surfaces which make contact normally only at their ends, the blocks increasing in thickness from their ends to intermediate portions thereof, and means for selectively pressing the blocks together.

l0. A rheostat including a plurality of superimposed carbon bloclis, each of said blocks having opposed flexible extensions on the surfaces thereof contacting adjacent blocks, said extensions being curved in such a manner that under lightpressure the only contacts between adjacent blocks are at the tips of said extensions.

1K1. A rheostat comprisinga plurality of superimposed carbon blocks, each of said blocks having a portion of relatively low resistance and a flexible portion integral therewith and arranged to normally contact a corresponding flexible portion on an adjacent block through a relatively small initial contact area under light pressure, said flexible portion of each block decreasing in crosssectional area from said portion of relatively low resistance toward said initial contact area to provide a progressive increase in resistance and flexibility, whereby under increased pressure an even and progressive increase in the area in contact is obtained.

In testimony whereof lhereunto afiix my signature this 24th day of September, 1927.

TTO W. GREENE. 

